Storage batteries with zinc (Zn) as negative electrode include zinc nickel batteries, zinc silver batteries, zinc air batteries, and, zinc manganese dioxide batteries. The disadvantage of these rechargeable batteries is short cycling life. The Zn products during discharge in these batteries are high soluble in the alkaline electrolyte of the battery. Therefore, during a battery's charge-discharge process, Zn repeatedly dissolves in the electrolyte solution and then precipitates on the electrodes. However, precipitation of Zn does not necessarily occur at the same location as where the Zn was dissolved. This results in the redistribution of zinc on the electrodes. The active material migrates from the edges of the electrode and congregates towards the center leading to the phenomenon called “distortion” or “shape change” of the zinc electrode. During cycling, this distortion of the zinc electrode gradually decreases the actual surface area, reducing the battery's capacity and shortening its cycle life. To limit this distortion phenomenon, efforts have been made to limit the migration of the zinc products during discharge or decrease the solubility of zinc products during discharge in the electrolyte solution. In addition, there is also research directed towards changing the non-uniformity of current density distribution on the electrodes. When current passes electrode, electrode polarization, i.e., the deviation of the electrode from its equilibrium potential, occurs. This electrode polarization effect increases with the increase in the current density of the electrode. It is equivalent to attaching an equivalent resistance on the surface of the electrode where the magnitude of the equivalent resistance increases and decreases with the magnitude of the electrode polarization.
The degree of polarization of a material for an electrode indicates the sensitivity of that material to changes in current density. When the current density in a material with a low degree of polarization increases, the polarization effect will not be apparent. On the other hand, when the current density in a material with a high degree of polarization increases, that material will be highly sensitive to the changes and slight changes in the current density will have significant polarization effects. Since the initial current distribution on the surface electrode is non-uniform, when the polarization of the zinc electrode is increased, the magnitude of the polarization effect and the corresponding magnitude of the equivalent resistance will also be non-uniform on the surface of the zinc electrode. Areas with high current densities will have high polarization and equivalent resistances while areas with low current density will have low polarization and equivalent resistances. This will result in the redistribution of the initial current density toward a more uniform actual current density on the surface of the electrode. Therefore, the distortion of the electrode can be reduced by increasing the uniformity of the distribution of the current density. Adjusting the equivalent resistances at different areas of the zinc electrode can achieve this uniformity. However, adjusting the equivalent resistances requires a relatively high degree of polarization. The oxides and hydroxides of lead, cadmium, thallium and indium can efficiently reduce the distortion due to a substrate effect. Lead oxide, indium hydroxide and thallium oxide reduce the distortion by increasing the degree of polarization and improving the distribution of the current density. After charging, the oxides of these metals in the zinc electrodes do not participate in the discharge process. They remain in a metallized state, and, during the next charging cycle, function as substrates for the reduction of zinc, producing a zinc product having a high degree of polarization. In contrast, mercury oxide increases the rate of distortion because the products during the charging process have a reduced degree of polarization. However, lead, cadmium, thallium cause severe contamination to the environment, and indium is an expensive metal. Therefore, in order to be environmentally friendly and have low production cost, it is necessary to find another method to improve the distribution of current density.
One method is to increase the layers and thickness of the separators at the edges of the electrodes that are in contact with the positive and negative electrodes. This increases the resistance between the positive and negative electrodes at their edges. This configuration lowers the current density at the edges of the electrodes and results in a more uniform distribution of the current density over the entire electrode. Currently, uniformity in current density distribution is improved by wrapping additional layers of separators at the edge of the positive electrode such that there are fewer wrapped layers at the center than at the edge of the electrode. However, this current method is only suitable for large and medium sized cubical or rectangular batteries. It is not suitable for use to fabricate smaller cylindrical batteries made by the winding method. For these small size cylindrical batteries with electrodes that are narrow and long, it is difficult to wrap a layer of material on the edges along the length of the electrode. Furthermore, after wrapping, the two electrodes cannot be wound. Moreover, this method of wrapping using micro-pore membranes that are 0.02˜0.04 mm thick is cumbersome and inefficient.
Due to the limitations of the prior art, it is therefore desirable to have novel separators for storage batteries that are simple, low in cost, easy to manufacture, and can reduce the distortion of the zinc electrodes during cycling.